1. Technical Field
The present invention relates to a vehicle seat belt retractor, and relates particularly to a seat back mounted retractor in which withdrawal of belt webbing is blocked in response to tilting of the seat back in excess of a predetermined amount.
2. Description of the Prior Art
A typical seat belt system used to restrain a vehicle occupant includes a length of belt webbing wound on a spool of a seat belt retractor. The belt webbing is extensible about a vehicle occupant to restrain the occupant. In the event of sudden deceleration such as occurs in a vehicle collision, a vehicle deceleration sensing assembly in the retractor is actuated to block withdrawal of the belt webbing from the retractor. If the retractor is mounted in a reclinable vehicle seat back, the sensing assembly should be operable when the seat back is tilted to different positions. U.S. Pat. No. 5,495,994 discloses one such type of vehicle deceleration sensing assembly.
A reclinable vehicle seat back has a nominal "installed position" as determined by the vehicle manufacturer. The installed position is a particular position to which the seat back may be reclined or tilted about the seat back adjustment axis, within the overall seat back adjustment range (forward and rearward tilting) of the seat back.
Some motor vehicle safety standards require that a seat back mounted retractor must automatically lock (prevent belt withdrawal) when the seat back is tilted forward or rearward to a position at least a certain number of degrees from the installed position. Such standards also specify that the retractor must not automatically lock when the seat back is tilted to a position within a smaller number of degrees from the installed position. For example, one standard specifies that a seat back mounted retractor must automatically lock when the seat back is tilted to a position more than 27 degrees from the installed position, but must not automatically lock when the seat back is tilted to a position 12 degrees or less from the installed position.
The retractor shown in U.S. Pat. No. 5,495,994 has an inertia sensing mechanism which includes a ball supported for rolling movement on a support member having a ramp surface. The support member is gimbaled so that it swings to maintain a predetermined upright orientation during tilting movement of the seat back. Upon sudden vehicle deceleration, the ball rolls up the ramp surface to push a lock lever into locking engagement with a ratchet. In order to achieve a tilt locking feature, a retractor constructed in accordance with U.S. Pat. No. 5,495,994 has stops which limit swinging movement of the support member upon substantial tilting of the seat back. After the seat back is tilted far enough that the ramp hits the stop, a further increment of tilting of the seat back causes the ball to roll far enough up the ramp to push the sensor lever into engagement with the ratchet and, thereby, lock the retractor and keep it locked. If the seat back is tilted to a position in which the support member engages the stop, but which is not tilted enough to actuate the tilt locking feature, the ball will roll to and stay in a position part way up the ramp surface, even when the vehicle is not experiencing sudden vehicle deceleration. When the ball is in this non-centered position, the inertia sensing mechanism is closer to a locking condition. Sudden car movements, such as can result from road shocks, which would not normally be large enough to actuate the inertia locking mechanism may, in fact, do so.